Field of the Invention.
This invention relates to polymers, compositions and methods for removing suspended solids in continuous production processes. More particularly, this invention discloses silicon-containing polymers flocculants for removing suspended solids in alumina production process streams.
Description of the Related Art.
Bauxite is the basic raw material for almost all manufactured aluminum compounds. In the course of production of aluminum compounds, bauxite can be refined to aluminum hydroxide and subsequently to alumina by the Bayer process, the Sinter process, and combinations thereof. Bauxites are typically classified according to their main mineralogical constituents as gibbsitic, boehmitic and diasporic. The mineralogical composition of bauxite can impact the method of processing.
During the Bayer process, raw bauxite ore is first heated with caustic solution at high temperatures to produce dissolution (digestion) of most of the aluminum-bearing minerals and give a supersaturated solution of sodium aluminate. Examples of the aluminum-bearing minerals include but are not limited to diaspore, the alumina trihydrate gibbsite, and alumina monohydrate boehmite. The resulting concentrations of dissolved materials are very high, with sodium hydroxide concentrations typically being greater than 150 g/L and dissolved alumina typically being greater than 120 g/L. Any undissolved solids known as the “red mud” solids, are then physically separated from the aluminate solution. A polymeric flocculant is often used to speed the removal of the fine solid particles. The red mud solids are principally composed of iron oxide but can contain other insoluble minerals such as complex aluminosilicates, sodium and calcium titanates, and hydroxyapatite. The residual suspended solids are removed by a filtration step. The filtered clear solution or liquor is cooled and seeded with alumina trihydrate to precipitate a portion of the dissolved alumina. After alumina precipitation, the solution (also called spent liquor) is reheated and reused to dissolve more fresh bauxite.
The Sinter process is an alternative or adjuvant to the Bayer process, and is commonly used for the treatment of high silica-containing bauxites. In the Sinter process, bauxite (or “red mud”) is calcined at 1200° C. with soda and/or lime prior to leaching with NaOH solution, generating sodium aluminate liquor (also commonly referred to as “supersaturated green liquor”) and insoluble “sinter mud”.
In many parts of the world bauxite is primarily composed of iron oxide and aluminum oxide, with only a minor amount of silicate minerals. Such bauxites therefore have a fairly low level of silicon as measured by SiO2 content. Typically, these bauxites have a SiO2 content of less than 5% SiO2. Still, in some parts of the world, the level of silicon-containing minerals in the bauxite can be higher. In such bauxite the SiO2 content can be, e.g., 5% or higher, and even as high as 10-20% SiO2. This higher SiO2 content is due to the presence of silicon containing minerals in the bauxite, such as kaolinite, illite, pyrophilite, chamosite or silica in its various forms (e.g., quartz). The caustic solution used to dissolve the aluminum minerals in the bauxite also dissolves part or all of the silica content therein, especially silica present in the form of aluminosilicate clays.
In the digestion step of the alumina recovery process, silica rapidly dissolves to form solutions supersaturated with silica. This dissolved silicate reacts with sodium aluminate or calcium aluminate in solution to form complex hydrated sodium or calcium aluminum silicates, part of which precipitates out of solution. Silicate mineral precipitants present in alumina recovery process liquors include cancrinite, sodalite, katoite, garnet, andradite, and various forms of calcium aluminosilicates.
Separation of undissolved suspended “mud” solids such as complex aluminosilicates from the alumina stream is an important part of the process and ensures that the precipitation step in the alumina recovery process can be carried out efficiently, resulting in a high quality alumina trihydrate product. Removal of suspended mud solids from a continuous process stream of red mud can be accomplished with the use of flocculants, which act to agglomerate the suspended solids causing them to settle more quickly. Historically, this flocculation step has been performed using polymeric materials such as starch and polyacrylate polymers. More recently, hydroxamated polymers have been used as Bayer process flocculants, such as those described in U.S. Pat. No. 4,767,540.
U.S. Patent Application Publication No. 2008/0257827 further discloses the addition of silicon-containing flocculants to Bayer process streams, particularly in combination with polyacrylate, hydroxamated polyacrylamides or poly(acrylamide-co-acrylate), for reducing solid content such as red mud present in Bayer process streams. Silicon-containing polymers disclosed in the '827 publication were prepared by various methods, including preparing a precursor polymer containing vinylformamide units, hydrolyzing them to form vinyl amine units, and then reacting with a silane agent to attach a silicon-containing pendant group to the amine group.
Still, there remains a need for an efficient way to remove much more of the unflocculated solids that remain suspended after use of current flocculation techniques since removal of such unflocculated suspended solids, which are currently considered very difficult to flocculate, involve the use of other solids/liquids handling step(s) such as filtration. In particular, there is needed flocculants able to remove these remaining unflocculated solids so that the filtration step is eliminated or at least less down time in cleaning the filter is required.